The present invention relates to a low-frequency power amplifier, in particular of the integrated type, with high stability.
In power amplifiers, most often having as a load a loudspeaker (e.g. being part of an acoustic box) or a servomotor for control systems, or an electromechanical transducer, the power to be supplied by the amplifier can be considerable. The amount of this power does not depend only on the limits of current, of voltage and of the power of the active device, but also on the need to limit the distortion introduced by the load which generally is not a linear element. The practical problem is thus to obtain the maximum useful power with the smallest distortion and with the smallest dissipated power in the active device, so as to minimize, on one hand, the cost, and on the other the dimensions and the weight, as well as the power supplied by the battery.
The problem of the stability of low-frequency power amplifiers is commonly solved, in the prior art, by acting on the voltage amplifier which drives the actual power stage, controlling its frequency response so that the latter can have a pattern which will ensure an adequate phase margin, which is normally greater than 60.degree., to the complete feedback system. For this purpose, a compensating capacitor is used. With suitable circuitry, it is possible to use a low-value capacity, e.g. between 5 and 30 pF, which can, possibly, be integrated.
On the other hand, the presence of the final power sections requires another compensation, made necessary by the fact that these sections have a relatively high loop gain and are feedback with a unit gain.
Their frequency response, with open loop, as is known, significantly depends both on the characteristics of the load and the output voltage.
These elements, besides the characteristics of the transistors used, may take the poles of the transfer function of the power sections into regions where instabilities can occur.
In order to eliminate these oscillations, the prior art generally makes use of a series R-C network, connected in parallel to the load.
In such a solution, R normally has a value ranging between 1 and 8 ohm, and C ranges between 0.047 and 0.22 uF.
This external network acts so as to reduce the open-loop gain of the entire power amplifier at high frequencies, damping the high-frequency oscillations.
It is easily understood that the high value of the compensating capacity C prevents the integration of the component in an integrated circuit.